Actually, maidenhair ferns are really hardy - just keep watering and they'll resprout in no time 🌱
(Just cut off the dead leaves before the new growth pops up, as it's a lot harder when the dead leaves are intermixed with the live ones, and spoils the look)

Water it very sparingly, otherwise it will just rot again. Also, it's not getting enough light at the moment, which is why it's "elongating" at the top. Try to find a different spot where it gets more light.

Then plant it into fresh soil (don't bury the bottom more than 1cm or so - rather prop it up with something). Discard the current soil and remaining cactus roots - they're unusable.

You'll need to take a sharp clean knife, and cut off the rotten bits until there's only healthy light-green flesh left. Leave that for a week or so to "scab over" (a light-grey "skin" covers the cut area).

It looks like your cactus is rotting at the bottom. If it's wet-looking, brown, soft and slimy where it meets the soil then that's the case.
Just propping it up again isn't going to help, because the rot will spread and kill the entire cactus.