Be murdered by the locals since I can't speak a word of Latin or 5th-15th century English. Plus I'm naked, without any sort of family connections, and unable to perform peasant labor.
If I somehow survive, and learn the language, and reach a position of influence, my medical knowledge isn't much help since I don't actually know how to create sulfa drugs (Where do I find pure ammonia (not uric acid) and sufanyl chloride?) or how to cultivate penicillin.
On the chemistry front... again, I don't know the practice for creating the pure compounds needed to do anything cool. I could whip up some poor quality black powder, but I don't understand the corning process well enough to make good quality stuff, and there's no way I could get to gun-cotton.
Physics is easy, but we're dealing with people in between the fall of the Roman Empire and the Renaissance. So there's not much call for physics equations, especially lacking a reliable way to turn them into weapons.
Mechanical engineering might help, but again, we lack materials.
Metallurgy would be good. I'm not a trained metallurgist, nor am I a blacksmith. The things I could do all require machines and materials not available.
I don't know the history of the time in enough detail to act as much of a fortune teller. Do you have the ability to convert dates between Julian Calendar and Georgian Calendar and Roman Calendar, while accounting for the skipped days in each? And do you have a memorized table of eclipses or important celestial events during that thousand years? I sure don't.
I could build a compass... assuming I could get an iron needle. I could build a sextant, but I would require precision manufacture beyond my personal capabilities, AND I can't actually use one properly without a set of tables that I don't have memorized. Plus there's not a lot of help in knowing your latitude when no maps use them.
My knowledge of architecture certainly isn't up to maintaining Roman arches or designing cathedrals.
Even if I learn to speak the local language, I probably can't write it. Even if I can write it, being a scribe is not exactly a high ranking or high impact role.
I can't draft, much less build, a schooner rig for a seagoing vessel, and while I think I could throw together a lateen rig, I doubt it would be efficacious enough to draw attention. Besides, I'm no sailor.