It could be that an initialization file was damaged somehow.
Simple test: hold down the SHIFT key as Mathematica starts.
More extensive clearings are system-dependent. See http://support.wolfram.com/kb/3274
If none of those work, you may send an example notebook, detailed instructions, the operating system level, and
the output from running SystemInformation in Mathematica, to firstname.lastname@example.org.
Refer to "Student Forum 29451".