Clutch was replaced and now it won't shift. What do I need to check?

First, check behind the driver's floor mat for brake fluid leaking from the clutch master cylinder. Also, take a flashlight and look for leakage where the pushrod goes into the master cylinder. Second, check for brake fluid leaking from the clutch slave unit. Have someone depress the clutch pedal and look to ensure the slave cylinder's piston moves the release fork enough to disengage the clutch. The problem is likely in the hydraulic system. Clutch fluid should be changed every 2 years. LuK are widely respected clutch master cylinder and slave unit replacement parts.