Gmod startup error

[oc2550]

I go into my steam account and try to launch garry'smod but it comes up with an error saying only one instance of the game can be running at a time.
I have tried restarting my comp and tried to delete and reinstall but it comes up with another error saying to close gmod before deleting local files.
I have searched my process list to see if gmod is running. I have deleted all apps that I have recently downloaded.

[SageLeopard]

The same thing is happening to me but all i did was reinstall gmod and it worked and now one month later its broken...anyway can I permanently fix it?

[Harlequin1212]

I get this error know how to fix it?

Problem signature:
Problem Event Name: AppHangB1
Application Name: hl2.exe
Application Version: 0.0.0.0
Application Timestamp: 4d094ebe
Hang Signature: ac7d
Hang Type: 2048
OS Version: 6.1.7600.2.0.0.256.48
Locale ID: 3081
Additional Hang Signature 1: ac7dd52a5e81b919470c18016f907cb7
Additional Hang Signature 2: 4208
Additional Hang Signature 3: 42084a2721082237d8e290f9796c8535
Additional Hang Signature 4: ac7d
Additional Hang Signature 5: ac7dd52a5e81b919470c18016f907cb7
Additional Hang Signature 6: 4208
Additional Hang Signature 7: 42084a2721082237d8e290f9796c8535