Ducane furnace I cleaned my furnace, replacing the filter as usual, but now the fan will not go on. It heats up, but no fan. Did I screw it up somehow?

Make sure all your dampers are open, air filter is clean, blower motor running as it should. Make sure main burner is actually on.

Hit the side of the furnace. Some times there's a sensor inside that gets locked up from a dirty filter.

next to the filter is the fan the cover for the fan should come off maybe two screws or lift off now the fan should be in what looks like a upside down snail the fan is in there , Now you can blow the fan out with blow on a vac or at the top of the snail there are some bolts you can undo and slide the fan out watch the elce lead on the fan. you can then clean this with a vac and paint brush. This a general view of how to clean a fan you should be able to work it out from this .

Dirt is the biggest enemy of your furnace. It can waste fuel and drastically lower efficiency. Dirt affects all three basic components of your furnace, so cleaning is the most important part of regular maintenance. The three parts of the furnace should be cleaned: the filter system, the blower, and the motor.

The furnace filter should be replaced or cleaned at the beginning of the heating season and about once a month during periods of continuous use. To check the filter, take it out and hold it up to the light. If it looks clogged, replace it with a new filter of the same type and size regardless of the length of time it has been used.

Step 4: Clean blower assembly, belts and pulleys to blower, and motor housing. Cleaning blower is critical if furnace has a squirrel-cage fan, because openings in this type of blower often become clogged with dirt. To clean blower, remove panel that covers filter to gain access to blower or panel on front of furnace. This panel may be slip-fit on hooks or held by series of retaining screws. Access to inside of blower is usually gained by sliding out fan unit, which is held on track by screws.

If power cord to fan assembly is not long enough to permit fan unit to slide all the way out, disconnect cord. Mark wire connections first so you'll be able to reassemble unit correctly. With toothbrush, clean each fan blade and spaces between blades. Then, with vacuum cleaner hose, remove all dirt and debris loosened by brushing. Vacuum belts and pulleys. Wipe motor housing clean to prevent heat buildup in motor.

Try the fan in the ON position on the thermostat, do you mean the fire shuts off or the blower?

see this causes and fix it. God bless you
If the furnace won't run, check the electrical service panel for a blown fuse or tripped breaker. Also make sure the power switch on or near the furnace is turned on. Check the unit for a reset button and try it. Some units have a built-in breaker or fuse panel.
If there is not enough heat, raise the thermostat setting five degrees to see if it turns the unit on. If the unit still does not generate enough heat, check the comfort controls . Clean or replace a dirty filter, clean the blower assembly (see the Forced-Air Distribution Fix-It Guide), and make sure that all registers are open and not blocked.
If the furnace turns on and off repeatedly (cycles), clean or replace the filter; and clean the blower assembly (see Forced-Air Distribution Fix-It Guide).
If the furnace is noisy, make sure access panels are mounted and fastened securely; spray squeaking belts with fan-belt dressing and replace worn or damaged belts; adjust the blower belt; lubricate motor and blower oil ports.
If the element does not heat, refer to the Heating Element Fix-It Guide for guidance.

Pressure switch is bocked inside try a small drill bit to clean down the connection tubes and blow it out after , check hoses are not blocked or split ,

when was lat time serviced --tuned up or cleaned --

open fan door and measure filter holder on inlet side of fan. it may also be in the return air opening on floor or celine

A short cycle heat cycle indicat a pressure switch replacement was needed. This did not fix the problem. the problem was in PCV line since the end was filled with slugs and cause the line to backup with water. When this happen the system would stop asking for gas. the water would leak out and it would restart. The problem was fixed by cleaning value at end of PCV. This released the water, two quart of water, that was back up causing problem. Curious if a filter change indicator was providing a clue that was missed?