Diagnosis

A houseplant with pale, washed-out leaves, fine webbing on leaves or stems, and yellow leaves

Pale, washed-out leavesFine webbing on leaves or stemsYellow leaves

Based on what you've described, these are the likeliest causes — ranked. Each one carries a tell-tale sign that distinguishes it from the others, and a single-minute check to confirm.

1Most likely

Spider mites

A tiny sap-sucking arachnid that lives on leaf undersides and breeds fast in dry indoor air. They're nearly invisible individually but leave diagnostic fine webbing across stems and leaf joints once the colony is established.

Tell-tale sign
Fine silken webbing strung between leaves or at the base of new growth, visible in raking light.
60-second check
Hold a sheet of white paper under a leaf and tap. Specks that start moving are mites; specks that don't are dust.
2Also possible

Too little light

The plant is getting less light than it needs to sustain the leaves it currently has, so it's stretching toward the nearest window and cannibalizing older growth. Low light problems show up in weeks, not days, which is why they're easy to miss.

Tell-tale sign
New growth is smaller and paler than the old leaves, and stems elongate between leaves rather than filling out.
60-second check
Hold your hand a foot above the plant at midday. If your shadow is soft-edged or barely visible, the spot is too dim.
3Also possible

Overwatering

The soil has stayed wet for too long, suffocating the roots and weakening the plant from the base up. It's the most common reason houseplants decline indoors, and it looks deceptively similar to thirst: a wilting plant in soggy soil is almost always drowning, not dry.

Tell-tale sign
Soil is wet more than an inch deep, and the lower leaves yellow or soften before the upper leaves change.
60-second check
Push a finger two inches into the soil. If it comes out cool and damp, the plant doesn't need water; it needs to dry out.
Start a new diagnosis
Canonical combo: pale-washed-out-leaves--webbing-on-leaves--yellow-leaves