What kind of computer? What version of Mac OS X? Please provide more information -- there are a few hundred different problems that can cause a hang at the gray screen -- simply saying your unknown system hangs at the gray screen isn't nearly enough information to troubleshoot the problem.
Have you repaired the disk using Disk Utility or DiskWarrior or some disk repair program? Have you repaired permissions? Have you made any software or hardware changes to the system recently? What exactly happened before and after the first time you experienced this problem?