Try booting from something else, like a USB drive, or even optical install media. In case your existing OS install has gone bad.
When you hold down Option, do you get a full boot picker screen, without corruption? Assuming the screen corruption happens in a later stage of boot, you could boot verbose and record the screen. See where it craps out in the boot process.
Some more specs on the system could help. For example, if you're booting from an SSD, it may have been unpowered long enough for data corruption. SSDs are rated to retain data for 1 year without power. Anything beyond that is a bonus. Hard drives are subject to mechanical problems and magnetic fade, both of which can crap things out after a long time in storage.