Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Go to emergency care for new loss of bladder control or urinary retention together with severe back pain, new leg weakness, numbness around the buttocks or genitals, or loss of bowel control.
These can be signs of pressure on the cauda equina nerves and require immediate assessment.
This is an emergency.
If trained to do so, sit upright and check the catheter for kinks while emergency help is being arranged.
And for anyone, someone who:
Seek urgent medical help if you cannot urinate and your lower abdomen is becoming painful or swollen.
Prompt treatment is also needed for fever with pain in the side or back, vomiting, confusion, or feeling significantly unwell, as these can occur with a kidney infection or blocked urinary drainage.
And for anyone, someone who:
Arrange a routine medical review if you develop new leakage, increasing urgency, difficulty emptying, repeated infections, catheter problems, blood in the urine, or a clear change from your usual bladder pattern.
Also seek review if your current plan is difficult to manage or is limiting daily activities.
The article also says
People with a spinal cord injury, especially at or above the T6 level, can develop autonomic dysreflexia from a full bladder or blocked catheter.
Warning signs include a sudden pounding headache, heavy sweating or flushing above the injury, goosebumps, blurred vision, anxiety, or very high blood pressure.
Every sign above is quoted from the sofidex article on Neurogenic bladder. If you are not sure, err upward — it is always reasonable to be checked.