New power supply installed, now receiving boot disc error

Not sure if anyone will be able to direct me on what could possibly cause this error, but I'll fill you in on what's been done.
This is concerning my mother's HP desktop, Window 7 Home Edition (I believe). She had been having issues with the computer randomly just shutting itself down. Sometimes after several minutes of use, sometimes after an hour of use. We decided it must be the power supply going bad and decided to have it replaced.
Took it to my BIL (as he used to work on computers for a living years ago) and he replaced the power supply. He didn't have a spare monitor to check functionality after putting in the new supply, but determined that the power supply was working as it should.
Brought the computer back to my mother and she hooked everything back up (mouse, monitor, keyboard, etc., etc). After starting it up, it came up with the "Boot Disc error" message.
I came over to her house to see if there was something that she may have done differently , but couldn't see anything telling. I was able to boot into the bios, but as computer illiterate as I am, that didn't really tell me anything.
So what I am wondering, would it have been possible for my BIL to have hooked up the new power supply incorrectly (i.e power cable to wrong ports??) and caused this error? Could the boot order/sequence make a difference?
My biggest fear is that she won't be able to get any info off that computer (she has many pictures and family research saved on it).
Hoping someone can enlighten me on what to do.
Thanks for taking the time to read this and I'd be happy to supply any further information if needed!

 

Thank you Steve for your reply. One of the things I wondered about was the boot sequence, I'm not sure it's even set to start with the hard drive? (Noticed the boot order when I went into BIOS). I would have tried to change the order, but am a dunce when it came to that and I wasn't sure how to even go about doing it! I may have to do more investigating!

 

No updates as of yet. I haven't been able to bring it back to my BIL to have him check it. My mother is currently using a "spare" computer at this time, so she's not in any hurry either. I was going to hook it up here at my house to check on it, but discovered I didn't have a spare monitor connector....have got one ordered and waiting for that to arrive and will do my further "investigating" at that time! Thanks for your concern. I will probably be back with further questions!

 

Oh dear, this is not looking good! I was able to get my spare monitor hooked up to my mother's computer and got into the bios settings. I did figure out how to change the boot order (set HDD) as 1st, saved the changes and exited. Still received the "boot disk error" message.
Neither her or I have a disk for this purpose and not sure where to even get one (if that would help??). Now I'm beginning to wonder if it wasn't her hard drive failing and not the power supply! She had never (to my knowledge) done any backups on her computer, so if the hard drive is/has failed, I imagine she's out of luck trying to recover anything on it?
Hope someone can offer some advice or a prayer if needed! :-( Thanks!

 

Make sure that there isn't a disc in the DVD drive.

 

Thanks all for your replies. I did check to make sure there wasn't a disk in any of the drives, didn't find any and didn't expect to as she seldom, if ever, uses it. I also checked her "media" slots as she used one of them quite frequently to upload her pictures from her camera to her computer.
Not sure of the specs on that computer other than I know it's a HP Pavilion a1268c. Has an AMD processer 64x2 dual core. The graphic is NVIDIA GeForce 6150LE. Has 1024mb memory, hard drive is 200GB. The optical drive is super multi DVD Burner w/lightscribe technology and DVD-ROM drive. (This was all taken from the front plate cover on the computer).
I believe she bought this computer used (many years ago?) as it states it was originally equipped with "Windows XP Media Center Edition". I'm guessing the prior owner upgraded it to Windows 7 (might be Home Premium??) before selling it?
She has many times stated that she'll never use Windows 10 (can't say I blame her, I'm not a fan either!lol) so don't think she'd even consider upgrading to one that has that. She's 82 years old and says that Windows 7 will probably outlast her anyway.
As far as connecting her hard drive to another computer to see if any data could be recovered, I think that's beyond my realm of trying to help. I'm hoping to get in contact with my BIL to see if he has time to look at it again.
Once again, that you for all your help/replies!

 

What I have in red and I am sure others will agree. Win 10 is the best thing since 98 Easy to use etc.