Oh, not good. So, either your 5th gear pooped itself by chance just at that particular time OR they damaged something to do with the selector on top of gearbox OR they fitted the wrong box back to the engine when it was out and yours is sitting in their garage. I doubt there is much you can do about it though as I doubt you have proof 5th gear was working before the work and it would be their goodwill to repair it. Have you had the selector cables off to check for damage?