Gas ovens often malfunction when the bake igniter is burned out or in a weakened state. Identify the bake igniter by looking for a small rectangular element near the burner. The burner is a tube inside the oven with small holes to allow the gas tp create heat. Examine the igniter. If it turns red but is not getting very hot, or if it is not turning red at all, it will need to be replaced. Usually the igniter is the problem. If the issue does not improve after replacing the igniter, the safety valve will have to be replaced.
It may come as a surprise, but for gas and electric stoves, clock and timer problems can cause the oven to malfunction. If the broiler and oven aren't working, but the burners are, the clock may have been placed in timer mode or set up for a self-cleaning cycle. If the clock has a manual option or button, push it and try using the oven again. If this does not help, there may be a wiring, thermostat or selector switch problem.
Gas and electric ovens have thermostats. Thermostats are difficult to diagnose and likely will need to be handled by a qualified technician, especially for models with digital temperature settings. One trick to try to recalibrate the thermostat on older models with temperature knobs is to remove the oven temperature knob. Remove any screws near the pointer under the knob. Move the pointer to the right to increase the temperature setting for the oven. Each notch you turn the pointer will raise the temperature approximately 10 degrees Fahrenheit. Replace the knob so that it lines up with the proper temperature setting. A professional can calibrate the thermostat much more accurately, so call a professional for best results.
If you have experience, you may be able to diagnose and repair the problem with your oven safely. But if you are unsure of the problem or are uncomfortable fixing it, call a professional. Working with gas appliances can be dangerous, so protect yourself by getting help if necessary.