A Marshall valve state 8080 amplifier uses a tube preamp and a solid state amplifier to transfer an electronic signal from a guitar to a speaker cabinet. When you turn on the power switch, the preamp tubes warm up, and you will have sound coming from the speaker within 30 second. If you have no power, the preamp tubes will not warm up, and the amplifier will not work. Several different issues could cause this.
Inspect the back of the amplifier to ensure the power cord is plugged into the amplifier. The plug disconnects from the amplifier for storage. The plug can work loose from the amp because of the vibration when it plays. If the plug is connected to the amplifier, move to the next step.
Try a different outlet or place a known working item into the amplifier's outlet and confirm the outlet works. A faulty outlet will prevent the amplifier from coming on. If the test fails, move to the next step.
Remove the thread-in fuse cover located on the back panel of the Marshall amplifier by turning the cover counterclockwise. Pull the fuse out. If the metal wire in the fuse is broken, it is likely the issue. Replace the fuse and secure the cover onto the back of the amplifier.
Take your amplifier to a qualified Marshall repair facility. Amplifiers store power even if they won't turn on. If you are not a qualified repair person, you run the risk of electrocution if you open the amplifier casing.