In that case we might have some options- I was not about to start on debugging a corrupted copy of working rom hack that it would have been easier to just redownload.
You will want a copy of the original/base rom for this.
Rather nicely the GBA does not change locations of things during hacking (thinking filesystem rebuild- pointers are a different matter) meaning the changes you see are either corruption from undeleting or your changes. I can think of many ways it would break it but there are equally many reasons why it would continue to work (especially if it is a basic hack* so it is worth trying partial hacks (playable is one thing, recovered work is quite another).
*if it is pokemon and you used one of those hacking tools I would not hold my breath but most other roms (assuming you do not have to fight compression and/or repointing). On the other hand those tools are not the system itself so they might be able to recover your work (or at least view it for long enough for you to press control and C before reimplementing it).
Before we get onto that the first place to look is the header- in my example earlier of the first few bytes of the file are more often than not the thing to go (VBA will run with a bad header but it has limits) and your rom hack should not have touched the header (and if it has it should not matter if your replace it) so that is the first stop. GBATA to check (and possibly repair)
http://www.no-intro.org/tools.htm or more likely you do a bit of copy and paste from the base rom.
Assuming it is not a simple header repair from here you get to systematically work through you recovered file and overlaying any changes into the rom you are rebuilding like you would if you were using corruption to find/test something or trying to reverse engineer another hack/using region dupes to infer things.
