It’s the strangest feeling to pedal forwards and not get anywhere. If it’s just started happening, you may be able to temporarily fix it.
Freehubs are typically not designed for maintenance. Rather than trying to open one up on the trail, first try shock tactics. Remove the wheel from the bike, and remove the skewer. Lay the wheel flat with the cassette and freehub facing upwards. Protect the disk rotor if you have one. Now give the freehub area a good whack with a strong stick, along the line of the axle. This can help to free stuck pawls (the bits that engage when pedaling).
Thin penetrating oil applied judiciously to the freehub can have the same effect, but remember to replace the freehub or at least repack it with the correct weight grease as soon as you can. Usually freehubs rely on relatively lightweight grease. Some even use mineral oil.
If this doesn’t work, you will have to immobilize the cassette by attaching it to the wheel. Zip ties from cassette to spokes are not strong enough (unless you have industrial strength ones). Instead, jam a tool (wrench, Allen wrench) or other piece of metal through both sides of the spokes close to the hub, and into the back side of the cluster. Zip tie this tool in place. Ride GENTLY – push off with your feet, bring your feet onto the pedals when you are up to speed, and then pedal in smooth circles (even strokes).
Changing gear is not recommended. Lowering the saddle will help to “hobby horse” – pushing the bike along with your feet flat on ground.
Remember to lift your feet clear of the pedals at speed, as you now have a fixed gear bike.