accidentally hit dont trust my comuter and now i cant undo it

This may work as well.

About the 'Trust This Computer' alert on your iPhone, iPad, or iPod touch - Apple Support

Change your settings for trusted computers

Your iOS device remembers the computers you've chosen to trust.

If you don't want to trust a computer or other device anymore, change the privacy settings on your iPhone, iPad, or iPod touch. Go to Settings > General > Reset > Reset Location & Privacy. Now when you connect to formerly trusted computers, the Trust alert will ask you whether you trust that computer.

I'm assuming you did this on your computer in iTunes. So in iTunes, sign out of your account, then sign back in and when you connect your iPhone you'll get the opportunity to trust your computer again.