Clutch not disengaging. If it was working when parked up check the fluid level in the clutch mastercylinder. Assuming there is sufficient fluid and the clutch pedal feels normal, your clutch plate may have stuck to your flywheel.
Try starting it in gear with the clutch depressed and the hand brake off, as it starts to move forwards stamp on the brake and is should free off.
Clearly this is not without its risks and you need to be prepared to switch off id it doesn't free off.
If the clutch has failed while the car has been in use, first check the fluid, but if that is OK, it is possible that the release arm has worn through and needs replacing.