My vulcan lg 300 deep fryer latch won't turn what should I do

Some fryer valves have a safety lock, usually a slide on the handle near the valve body that must be released to open the valve. Check for this first. It's very rare for a fryer valve to stick unless it was forced shut with something partially blocking it. If not, let the fryer cool completely. Dip as much oil out as possible. Then disconnect the gas line and, with help, turn the fryer upside down draining the remaining oil into a suitable container. At this point, you have little to lose. Take a large wrench or piece of pipe to get more leverage and try to force it open and free it up. Do not use so much force that you damage the kettle. If you get it free, check to see that it does not leak before putting oil back in the fryer. If it leaks or fails to come free, replace the valve.