Without knowing the model of radio or the specific repeater you are trying to access, the only things I can suggest are 1) check to make sure your radio is setup for duplex operation, 2) check to make sure the offset is correct for the repeater, 3) make sure you are in tone mode (not tone squelch mode) and that the tone is correct for the repeater, 4) if the repeater is still not coming up, listen for a while to see if others can access it. If you don't hear other traffic on the repeater there is a possibility that it is down.
Other information that would help get you an answer would be the make / model of the radio (you said Vertex but that could also mean Yaesu), and information on the repeater you are accessing (frequency, location, repeater callsign).
Answers & Comments
Without knowing the model of radio or the specific repeater you are trying to access, the only things I can suggest are 1) check to make sure your radio is setup for duplex operation, 2) check to make sure the offset is correct for the repeater, 3) make sure you are in tone mode (not tone squelch mode) and that the tone is correct for the repeater, 4) if the repeater is still not coming up, listen for a while to see if others can access it. If you don't hear other traffic on the repeater there is a possibility that it is down.
Other information that would help get you an answer would be the make / model of the radio (you said Vertex but that could also mean Yaesu), and information on the repeater you are accessing (frequency, location, repeater callsign).