Hi, it's probably because of the dirt that gets collected under the foreskin if it is left uncleaned for many days.
Next Steps
You can soak the foreskin for 5-10 min inside a cup filled with lukewarm water & try to retract the foreskin gently. Once retracted, use a slow stream running tap water to clean the dirt under the foreskin. Use a tissue paper or wipe off the extra dirt and allow the area to air dry.
Health Tips
Regularly clean the under the foreskin to remove the smegma and prevent further adhesions.