First, verify that you are using a center negative, isolated 9V power supply that provides enough current for your pedal.  The power plug should have 2.1mm inner diameter and 5.5mm outer diameter.  That is a standard size for guitar pedals, but not for generic (non-pedal) AC adapters.

Next, check the power cable and plug to see if it is damaged, dirty, or corroded.  The plug in the photo below is dented from constantly plugging and unplugging pedals on my desk, and no longer works reliably.  Power cables can be frayed by rubbing against sharp edges of pedalboards or chewed by pets.  If the plug or the cable are physically damaged, replace it with a new power cable.  If it is dirty, clean it using isopropyl alcohol or a contact cleaner designed for audio plugs and jacks (CAIG DeoxIT Contact Cleaner is one example).

If trying a few different cables does not fix the problem, the power jack may be dirty or damaged.  You can clean the center barrel and spring using isopropyl alcohol.  Allow it to dry completely before powering up the pedal.

The DC power jack has a cantilevered flat spring that contacts the outside of the barrel plug (positive voltage) and pushes the inside of the barrel against the pin (ground).  The spring can be damaged if the power plug was stepped on or the pedal was dropped with a power cable connected.  It is a standard DC power jack with 2.1mm inner diameter and 5.5mm outer diameter.  Replacements are available through most electronics distributors or pedal building supply shops.  One example is the Kobiconn 163-7620E-E (available from Mouser or Small Bear Electronics).  Any music electronics technician should be able to replace it, but you can also contact us to perform the repair.