Is your computer running XP and set up for automatic updates?
If it is try turning off automatic updates and do a system restore back to about 10 or 12 April.
I had exactly the same problem with my computer - the wireless ad hoc connection would appear to be working and did for internet connection but file sharing ceased to be available.
I tried reinstalling the network with no effect.
When I turned automatic updates off and did a restore to 10 April everything worked fine again.
G7SWZ