|Boot up with a Linux disk.|
Ubuntu.com is were to find a (free) ISO which download/burn to a dvd. Boot with that dvd.
You can then access the hard drive and safely copy all your files etc. to external storage; typically dvd but an external hard drive is nice(r) too?
Ubuntu (and other similar dvd Linux variants) will load into RAM only; nothing written to the hard drive - unless you tell it to install itself there (which DO NOT).
Once you have your data copied elsewhere (and checked the copies are accessible OK on at least one other working computer) you can safely set about resolving you actual boot up problems etc.?
What make/model is your computer? Possibly pholks here can help you get it back again - up and running as it were...
Two things to try - AFTER you have safeguarded your files etc. as above - try booting into safe-mode; similarly using a restore point to an earlier time?