New Post: Ciritical error when adding product to checkout or going to the checkout page

Perhaps try restarting the application pool and the application.

Also check the DNN event logs to see if an error is logged. If none is logged there then also check the server application log and see if you can find any clues there.

And of course, think of any upgrades or changes that may have been made to anything, including other modules, the DNN instance or the server.
Regards
ROb