If you use a password manager to generate a secure new password, you should copy it into notepad until you've confirmed that the manager has actually saved the password. I can hardly believe LastPass is so badly designed, it can sometimes forget the passwords it generates, leaving you locked out of your own accounts. Unbelievable.
I use LastPass and have not ran into this problem. LastPass will save generate passwords for you. If you delete it by mistake, then you can also recover it.
I think you can have unlimited recovering for the paid people (i.e., the people who pay $12 a year).
For free accounts, then LastPass will give you up to 30 days until it is unrecoverable.
If I'm right (30 days or unlimited), then you should be able to recover (assume you somehow deleted it).