Usually clogging will happen due to following reasons:
1. sensor is loose ( so temperature goes much higher than it is displayed on the screen ) resulting filament to clog
2. Temperature setting is wrong
3. Extruder cooling fan is broken or stopped ( not functioning)
4. Teflon linear is damaged due to high temperature ( if sensor is loose)
Please check and mount sensor as shown in attached image:
After this you can start heating take extruder to 250 degree centigrade, and then try to extrude with ABS filament.
If filament does not come out remove the inserted filament cut off the melted tip and extrude again.
If problem persists teflon liner will need to be replaced.
You can use the old nozzles and clean them by storing them in pure acetone in a closed bottle overnight.