Lauren, the short answer first: a three day dip after a feverish flu is one of the most ordinary things a feeding body does, and it is very unlikely your supply is gone. Between the fever, barely eating, dehydration, and feeding far less than usual, every lever that lowers supply got pulled at once, so a softer, emptier breast right now is the expected result, not the verdict.
Because milk works on supply and demand, the recovery is the same picture in reverse. Get frequent effective milk removal happening again, feed on demand aiming for the usual 8 to 12 times in 24 hours at this age, add a couple of short pump sessions if direct feeding isn't emptying you well yet, and drink to thirst. Most people see supply climb back within several days, and largely restored within one to two weeks. If baby is fighting the softer, slower breast, that grizzling is often just impatience with the reduced flow, so a calm relatch and a little breast compression usually helps more than topping up does.
Two things I want to name honestly, because a forum can't see your baby. First, watch wet nappies and nappy output over the next day or two: steady wet nappies mean milk is going in whatever your breasts feel like. If output drops off, or baby seems sleepy and hard to rouse rather than cross and busy, get them seen promptly. Second, if one breast becomes red, hot, and painful with the aches coming back, that can be mastitis rather than the tail of the flu, and that needs looking at rather than waiting out. Priya is exactly right that you never needed to pump and dump for flu, the guide to breastfeeding when you're sick covers why, and how to increase your milk supply has the step by step for rebuilding a dip. Come back and tell us how the week goes.
