When does the Shuttle Blast off again?

The SSMEs can be shut down on the pad as long as the SRBs have not ignited. This is called a "pad abort", and has happened five times, on STS-41-D, STS-51-F, STS-51, STS-55, and STS-68. It has always happened under computer (not human) control, caused by computers sensing a problem with the SSMEs after starting but before the SRBs ignite. The SRBs cannot be turned off once ignited, and afterwards the shuttle is committed to take off.