What model MBP? Was there any recent system modification you made?
1) Try booting into recovery mode. Run a disk check on your volume using disk utility.
Press C during startup
Start up from a bootable CD, DVD, or USB thumb drive (such as OS X install media).
Press D during startup
Start up in Apple Hardware Test (AHT).
Press Option-Command-P-R until you hear startup sound a second time. ⌘⌥PR
Press Option during startup
Start up in Startup Manager, where you can select an OS X volume or network volume to start from.
Press Eject, F12, or hold the mouse or trackpad button
Ejects any removable media, such as an optical disc.
Press N during startup
Attempt to start up from a compatible network server (NetBoot).
Press T during startup
Start up in Target Disk Mode.
Press Shift during startup
Start up in Safe Boot mode and temporarily disable login items.
Press Command-V during startup ⌘V
Start up in Verbose mode.
Press Command-S during startup ⌘S
Start up in Single-User mode.
Press Option-N during startup
Start from a NetBoot server using the default boot image.
Press Command-R during startup ⌘R
Start from the OS X Recovery System1