General:
* Blocked
* Hindered
* Frustrated
* Impeded
* Thwarted
* Hampered
* Obstructed
* Stalled
* Checkmated (more figurative)
More specific:
* Baffled (if it refers to mental confusion)
* Confused (if it refers to mental confusion)
* Paralyzed (if it refers to a complete stop)
* Deadlocked (if it refers to a situation where progress is impossible)
* At an impasse (if it refers to a point where no further progress can be made)
The best synonym for "stymied" depends on the specific situation and the nuance you want to convey.