Retaking the ACT is very common, and you can take it up to 12 times. Of students who took the test more than once, 55 percent improved their scores. Because you can choose which ACT scores to send to colleges, you can always re-test and then determine which scores to send. You can register to retake the test on act.org.
Many colleges and universities accept both ACT and SAT scores, allowing you to determine which of these you want to send. If you have problems getting a good score on the ACT, then try the SAT to see whether you can pull a more competitive score. The SAT measures knowledge areas different from those on the ACT, and the SAT does not have a science section. The SAT is a bit longer than the ACT.
Because the ACT is accepted in 50 states by nearly all colleges and universities, there are most likely schools that will accept even a lower score. If you already created a list of schools to which you want to apply, then try broadening your list to include less competitive schools that accept lower ACT scores.
Some schools do not require an ACT score, and these may be a good option if your score is extremely low. Community colleges typically take anyone, regardless of ACT score. You can always go to a community college for a few years and then transfer, which allows you to bypass the ACT process. In addition, some prestigious schools waive the standardized testing requirement, considering it not to be a good measure of one's academic abilities.