Spell Slots in D&D 5e: All You Need to Know!
See the multiclassing rules to determine your available spell slots. ... You regain all expended spell slots when you finish a Long Rest.
Warlock Multiclass Options: A Complete DnD 5e Guide
In D&D 5e (2014): Warlock Pact Magic spell slots are separate from the standard multiclass spell slot pool. You cannot use your Warlock slots to cast spells from other classes, and vice versa.
Multiclass Spellcasting Calculator: Dungeons and Dragons 5e
Calculator for the number of spell slots possessed when multiclassing, in Dungeons and Dragons, 5th edition.