Literally: It means you will have an accident (car accident, for example) but it is also slang for "I'm going to suddenly relax (because I'm tired)". Perhaps you can imagine you are walking very fast (because you are very busy) and 'crash' into your bedroom, which makes you sudddenly rest.

That means "I'm tired and about go get into bed," in its simplest form. Basically, so tired that you're going to "crash" into your bed.

it means i'll be going to to sleep soon.