Increase Taxes is one of your options when your city goes broke
open the Budget Panel
The city advisor will tell you:
"You have no money and you are in the red! All services cannot operate until you balance the budget! Options: (1) Turn Off Sevices (2) Raise Taxes (3) Zone RCI and wait for more taxpayers to move in"
To increase the taxes, use the "plus" signs near the tax rate. If you have a City Hall with a Department of Finance, you will be able to set the tax rate by zone and by wealth level.
Note that increasing the taxes makes the residents unhappy, so you should decrease them back to a reasonable level as soon as you can, otherwise the Sims might leave your city.
