[Silvanerian]

A fighter gets 1 decreased in thac0 per level. So a fighter level 1 has a base thac0 of 20, a fighter level 7 should have a base of 14, and so on.
Then you add your modifications, in your case:

Strength bonus: 18/54 gives +2 to hit.
Grandmastery: depending on mods etc, anywhere from +3-5 to hit. I assume +3 as you seem to play the vanilla game.
Weapon enchantment: whatever bonusses your weapon adds to hit.

Since you state base thac0 22 and actual thac0 17, which is a modification of 5, I assume you use a mundane longsword.

Edit: could be that grandmastery in vanilla game adds only +2 to hit - but I'm sure you get the picture.

Basically, the things that can lower your thac0:
Blindness, Glitterdust type effects
Level drain
Ability score (strength) drain


Check your character sheet to ensure your character does not suffer from any of these effects.
All the above effects will wear off over time, with the exception of Level drain and a certain type of blindness (druidic spell called: Natures beauty). Thus, try to wait a bit, alternatively try resting.

If all else fails, then I suggest you download shadowkeeper, look up your character and check what base thac0 is displayed - if it doesn't match what you should have, then edit it.