First, you need to identify your issue: is it textures or items. If textures, follow the answers above you.
In case of textures or items, and you just saved into wrong Iwad (Ultimate Doom instead of DOOM II) :
The easiest thing is what @Stabbey told you: Copy the whole map, open up a new map in different doom format, paste it, test it, save it if it works properly.
Here is what we understand from your post:
You saved a DOOM I (Ultimate Doom) map. As a result, you can only play it in Ultimate Doom.
If you want to play that map in doom 2,and Stabbeys solution did not help, you need to do as @Dark Pulse already said.
First you need to download slade. Unfortunately, the website for Slade currently says "connection refused."
In Slade you open up your wad file, find the marker that has E1M1(Or ExMx (Where X stands for number) as Ultimate doom is in Episode format) and change it to MAP01 (Anything between Map01 to Map30 will work, depending on which episode it's on). Then save the file and run the map then.
Or another (third) option (This also required Slade 3), is to add the missing textures or items directly from the Iwad. You need to figure out first, if you used Doom I (Ultimate Doom) and DOOM II textures, and then, which ones are missing.
After that, you open up your wad file in Ultimate Doom Builder or Doom Builder 2, look for the name of missing textures, write them down, and then copy them from Doom I (Ultimate Doom) Iwad into your file. To see how to add textures, look for doomkid tutorials. My texture issues were helped by this tutorial:
However, for adding items, you may need to search around.
When you have multiple sources listed as source for your wad, you will need to make sure all the textures are also present at the final product. If you test a wad with multiple sources on only one Iwad and have not compiled them into your Pwad file, you will get missing texture or item errors. The same is true if something is incorrectly added.
You can always include a wad file in your troubleshooting posts.
You can edit your post by clicking on Edit Post button at the bottom.
The backups are made automatically by Ultimate Doom builder, in case it crashes. It tends to still have a lot of bugs, since it is a community developed developer tool. Bugs are bound to happen, backups are neccessery to prevent loss of data.
Edit: @Stabbey I actually .zip archive my backups and different map versions until the development is complete, then I .rar it into highest compression possible. Despite prefabs, you never know when you need to find something from an old map.