Welcome to the forum. Thanks for being concerned for the snakes welfare.

I second the earthworm idea. Small night crawlers, maybe.
We would love to see a photo when you have a chance. It may also nail down the ID of the snake. Here's a link to the care sheet that might help.
http://www.thamnophis.com/caresheets...n_Garter_Snake

I think Stefan has the right idea. The snake is probably just coming out of brumation and needs sometime to adjust.
Keep us posted on your new friend.