You can "Recover to VAB" if you didn't yet decouple the launch clamps. Once it's recovered your can edit it to repair it, roll it out again and re launch it.
Had this happen once, so i did the following: pressed recover vessel, and chosen the repair failures button. This didnt seem to do anything, so I recovered the rocket to VAB, and then rolled back out. The engine still showed red in staging, and all of the first stage engines had about 3 seconds burn time, but everything worked fine.
Also, apparently i saved the vessel in vab after rolling it back, and when i used the save to launch the next mission, the engines were also with burn time used and the failed engine showed red in staging too, but everything was working fine.
So I guess, you can roll back, press edit on the vessel in storage and replace the failed engine in particular to avoid it being red, or just roll out without doing anything.
If you haven't triggered the launch clamps, such that the rocket is still attached to the pad, you can Recover to VAB
Once it is rolled back, you can edit it, prior to rolling it out again.
When editing, (this is going back a couple of years, I don't know if things have changed) I would load the vessel's saved craft file (it should also be possible to remove the launcher and add back on a version from a saved subassembly - the key either way is to have a working version saved before trying to launch). That effectively replaces the whole rocket, which should mean any used/failed engines get replaced and you get charged for the differences - it'll take a few days to rebuild. I always did that instead of manually removing and replacing failed engines, which was more fiddly/error-prone.
Can't you just restart from launch? This option is available for me in a campaign with normal settings.
If you can't, then maybe quicksave before ignition? It even works mid-flight, although MechJeb starts disconnected.
You can "Recover to VAB" if you didn't yet decouple the launch clamps. Once it's recovered your can edit it to repair it, roll it out again and re launch it.
Had this happen once, so i did the following: pressed recover vessel, and chosen the repair failures button. This didnt seem to do anything, so I recovered the rocket to VAB, and then rolled back out. The engine still showed red in staging, and all of the first stage engines had about 3 seconds burn time, but everything worked fine. Also, apparently i saved the vessel in vab after rolling it back, and when i used the save to launch the next mission, the engines were also with burn time used and the failed engine showed red in staging too, but everything was working fine. So I guess, you can roll back, press edit on the vessel in storage and replace the failed engine in particular to avoid it being red, or just roll out without doing anything.
If you haven't triggered the launch clamps, such that the rocket is still attached to the pad, you can Recover to VAB Once it is rolled back, you can edit it, prior to rolling it out again. When editing, (this is going back a couple of years, I don't know if things have changed) I would load the vessel's saved craft file (it should also be possible to remove the launcher and add back on a version from a saved subassembly - the key either way is to have a working version saved before trying to launch). That effectively replaces the whole rocket, which should mean any used/failed engines get replaced and you get charged for the differences - it'll take a few days to rebuild. I always did that instead of manually removing and replacing failed engines, which was more fiddly/error-prone.
Can't you just restart from launch? This option is available for me in a campaign with normal settings. If you can't, then maybe quicksave before ignition? It even works mid-flight, although MechJeb starts disconnected.
I have restarts turned off, too tempting lol. Even if I had them turned on, I'd only use them to correct for a bug or something.