Have you edited the view "product / cart" ?
I think that you have made a mistake when editing this file.
Can you try to save it's content in an external file and then delete the modifications (clicking on the dustbin in the views listing), try again.
If it's working so your changes were not correct, so you can do them again without forgetting the "proceed to checkout button"
Before anything, you can too check in the options of your cart module if the "proceed to checkout button" option is enabled.