it could be any number of reasons causing the problem.some more info would help a bit.
ie/operating system,name of isp,type of connection,browser used,any error messages displayed,new software installed lately etc etc.
if xp or me try a system restore to a date when all was well.
try clicking start/run and type "sfc /scannow" click ok and insert the windows cdrom if prompted(wont be prompted to do this if the os is pre installed)
windows 98 start/run type "sfc"
repair internet explorer click here click here click here
all i can suggest sorry.