We do not know why you received a check from your retirement plan----ask the custodian of the plan why. What kind of retirement plan? Did you leave an employer with whom you had a plan? When you leave an employer with whom you have a retirement plan you have an amount of time during which to roll over your retirement plan to a new plan or to cash it out---if you do not do anything then they might send you the money since you did not express your preference for what to do. How long ago did you receive the check? Not cashing it yet is not relevant. The money in a retirement account is taxable when you take it out of the plan---or when they send you the money unless you roll it over to a new plan before it is too late.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
