Multi-Programs in Groove Agent 4 only save the plugin’s parameters, the samples aren’t moved. If the samples are moved from the original folder or deleted, GA wont be able to find them.
Do the following to prevent this from happening:
-Right click each kit of your Multi, then click Export Kit With Samples. Pick a folder you wont delete.
-Go to that folder, then drag and drop the resulting .vstpresets into each GA kit slot. This will update the sample path used by your kits.
-Save the multi.
This is admittedly something a bit unintuitive in a drum sampler, but hopefully it will be improved in a future version.
Your samples are likely still somewhere in your hard drive. Try using Windows search to find them. If the samples were recorded with Cubase, they’re probably in a project folder.