It can be alarming when the batter for the cake you're preparing looks too soupy to do anything in the oven but crisp up a bit. If you're using a new recipe for what promises to be a moist cake, it's quite possible the appearance of the batter is correct - read the recipe again to see if makes mention of batter consistency. If it's a tried and true recipe, there are a couple tricks to use to thicken the batter.
Cooling, flour or custard mix
If it's possible - or likely - that the butter was too warm, place the mix into the fridge for a few minutes. After removing, let the batter return to room temperature and try mixing it again.
Try sprinkling 1 tablespoon of sieved flour to the batter and mix thoroughly - be sure to scrape the sides of the bowl. If this method seems to be working, you may add an additional 1 or 2 tablespoons until the batter reaches the desired consistency. Don't add too much extra flour or the cake will become useful as only a doorstop.
Before you give up on the batter and try again from scratch - or just run out and buy a cake - here's one last trick: try adding a tablespoon or two of powdered custard to the mix. Use a flavour that is the same as your cake mix or most suited to it. You might have found your fix. . . and a new ingredient to use for your cakes.
Check that the butter is not too soft before mixing. Check that dry ingredients have not been exposed to moisture. Sieve powdered ingredients to avoid lumps and uneven distribution. Don't set your electric mixer at too high a speed.