Resolved limited or no connectivity

Got a call from a friend with hughesnet internet and about 3 days ago he started getting the message limited or no connectivity. He called them and they claim everything is ok with their equipment. He switched to another computer that always worked before and got the same error. The only things common are the cable and modem? Any thoughts about how to check the modem? I'm not familiar with hughesnet so am not sure how to tell him to approach the problem. Machines are using XP Home but don't know anymore about them and as of yet don't know which model of modem it is.

TIA

 

Sorry for the delay in getting back here. I appreciate the information I received and have bookmarked the info.
Here is what happened. 1) He tried 3 different computers with the same results so I told him it was more than likely the sat modem. Hughes wouldn't consider that as a possibility.
2) He called them and told them to send someone out to either replace the modem or remove the equipment.
3)They sent someone out, replaced the modem and everything works fine.