Hello, my name is Dan.
I will do my best to help you, the only stupid question is the unasked one.
Unfortunately you seem to have passed a point that it will not boot from that disk without first repairing it. In fact it may not even be repairable.
Having said that there is hope, slim hope but hope nonetheless.
What happened is the internal hard drive that the OS is on along with all your stuff is full to the point that the OS cannot boot.
I assume that you saw warnings that the disk was getting full and you looked and thought that a few MB of room was enough.
It is not, OS X needs about 10% of drive capacity to operate without which it reaches a point that it locks up and all data can be lost.
Photos, music, documents, movies iTunes and iPhoto
The repair s both costly and time consuming. It will require , at a minimum, Disk Warrior.
That may be enough to repair the drive. It will also require an external hard drive with a bootable OS X that will run your Mac so that room can be made on the full drive.
This is the only way this can be fixed and save your data other than sending it to a data recovery service and that will cost much, much more.
If you have two Macs and a cable to connect them (like a Firewire/Thunderbolt, depends on what models you have) then it becomes a little easier.
That is the best answer I can give you, hope it helps.