Spells with a verbal component (most) requires that the caster be able to speak. If silenced, gagged, etc. the spell may not be cast. Verbal spells require knowledge of the Mage Tongue, which is learned by all spell casters before they can cast their first spell. Casters such as Sorcerers and Warlocks are able to use a verbal component, even if they don't understand the language.
Spells with a somatic component requires a hand gesture on the part of the spell caster. A somatic component can be done one-handed, but the free hand must be fully capable of motion. If paralyzed, you cannot cast a spell with a somatic component. If both hands are tightly bound (cannot move your fingers), the somatic component cannot be accomplished. Limited bondage, such as tied wrists that leave the fingers free, may allow somatic spells to be cast.
The D&D 5e rules system uses Verbal, Somatic, and Material (VSM) components for spell casting. Not all spells require a material component, but to cast those that do, the character must have a supply of that material on hand. In some cases, the material component is not consumed.
In those cases where material is consumed, there are two cases:
Common Material
Costly Material
In the case of a common material (pinch of sand, bit of spider web, etc), the player should equip the character with ten units of that material. Once equipped, it is assumed that these are periodically renewed. Since the cost is small, renewal is automatic. Thus, you'll never run out of this component.
In the case of costly material (powered silver worth 25gp, a gem worth 100gp, etc), the material is actually consumed. Players should track how much they have of this material. Once depleted, new materials must be purchased or otherwise acquired.