Does it work if you connect the computer to the router with an ethernet cable ?
Yes..It's works when i connected to the router tirh cable.
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Disable wireless security in the router temporarily. If wireless then works you know it's either an encryption issue (unlikely as it worked before) or could be a reception problem. If it still doesn't work with wireless security disabled it is very likely to be a reception problem.
You should try to optimise your reception and avoid interference.
1) raise the router above furniture level
2) Experiment with channels (some will work better or worse depending on your environment)
3) If you can detect strong neighbouring wifi, use a channel 5 stops away from strongest.
4) Relocate cordless phone base or video sender etc.
5) Be prepared to move the computer (or at least turn it so your body is not between the router signal and the wireless adapter's antenna.