Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Call local emergency services for severe trouble breathing, signs of a stroke, severe chest pain, loss of consciousness, or another possible medical emergency.
Seek immediate crisis support if you may harm yourself or someone else.
Arrange prompt medical advice for new or worsening pain, nausea, sleep problems, or other symptoms rather than relying only on guided imagery.
Contact the healthcare professional who recommended guided imagery during routine hours if it repeatedly causes anxiety, nightmares, flashbacks, dizziness, or worsening symptoms.
Also contact them if you are unsure whether it is safe to use alongside your current treatment.
The article also says
Guided imagery is not emergency care.
Every sign above is quoted from the sofidex article on Guided imagery. If you are not sure, err upward — it is always reasonable to be checked.