At 20th level, you can draw on your inner reserve of mystical power while entreating your patron to regain expended spell slots. You can spend 1 minute entreating your patron for aid to regain all your expended spell slots from your Pact Magic feature. Once you regain spell slots with this feature, you must finish a long rest before you can do so again.
RollQuest ships the D&D 2014 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.