Computer files don’t just delete themselves. Computers only do what you tell them to.
If you were working on it recently, you might still find it listed in GIMP by clicking File > Open Recent – if you can’t see it listed there, click on the “document history” option.
If you saved the image as an XCF then you could also do a file search in your computer – Click START > then in the search box type this: *.xcf
If you saved as jpeg or png, you could also search for these files using *.jpg and *.png
If the image file really was deleted, then you can’t get it back.
It should be where you saved it at. You could do a search (the entire drive) for the format you saved it as (Which should have been .xcf or .xcf.bz).
In the search input .xcf (or whatever format it is under, exempli gratia, .png, .tif, .jpg, .jpeg, &c.)