I think i forgot the unlock pattern or one of my friends may have changed it...

Hi you can follow the following steps to unlock it
I hope it will help you

Step 1. Make your LG GT540 Android Phone in Standby mood.
Step 2. Open the dialer of your LG Optimus GT540.
Step 3. Input 2945#*540# and press send.
Step 4. Input the provided unlock code.
Step 5. Click Unlock.

Enter in the incorrect pattern 5 times. It will then pop up with "Forgot Pattern?" at the bottom. Click it and input your google credentials (usually gmail name and password). You can then create a new unlock pattern.

Relax, you still can unlock lg gt540 screen pattern even you forgot it or your friend change it with your google account. But if you never setup google acount for your phone, you only can unlock lg phone password by reflashing that phone at mobile repair shop.