Some of the most common issues when dryer will not heat are:
Incorrect voltage? The motor in a dryer runs on 120 volts. The heating element works on 240 volts.
In-line fuses that might have blown.
Thermostats not functioning properly.
Bad timer for continuity of the switch contacts.
Bad continuity of the motor windings. Or grounded motor.
Broken or burned heating element or no continuity.
Bad temperature selector switch or no continuity.
Broken or loose wiring.
Clogged exhaust vent duct system.
For electronic control models, an error code could tell what the issue is.
Remember safety always your priority and always use professional and trained appliance repair technicians when dealing with electrical or gas appliance repairs !