Low hydraulic fluid will not prevent starting. The fuel filter shouldn't necessarily be full of fuel but if its empty my indicate lack of fuel flow. Look for debris in the fuel tank. A long skinny screw driver with a dab of grease on the end can be used to retrieve debris from bottom of tank. Remove the fuel line from the tank side of the filter and check for fuel flow. Blow into the line back into the tank then check for fuel flow. If it starts flowing then you will likely find a piece of debris in the tank. Reconnect the fuel line to the filter and remove from carb and be sure fuel is getting to carb.
If unit wont turn over. Check battery voltage: 12.6 volts or better. Be sure pto is off, unit is in neutral, and brake pedal is depressed. Check the fuse. Then with a test light check for 12 volts on the yellow or white wire on the small post of the starter solenoid with key in start position. If you have voltage to the solenoid but not to the starter then either the solenoid has a bad ground or is faulty and should be replaced. If no voltage to solenoid then you have a bad wire or switch between the fuse and solenoid. Voltage travels from fuse to ignition switch to pto switch to brake switch (to seat switch if it has four wire plug) to starter solenoid. You can use a test light to chase down where voltage stops.