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Nothing lights up

No power, no ground, a floating net, or a part that was never wired.

Is it running?

Press Run. A circuit that is not running is not doing anything, and the LED on a stopped canvas is drawn in its off state. The readout in the header starts counting when the simulation starts.

Is there a complete loop?

Current needs a way back. The commonest cause is a missing ground: a part connected to 5 V through a resistor and to nothing else has no loop, and an LED in it stays dark.

A net with nothing driving it is Floating, and the editor warns about a node that is not connected to anything when the circuit is built.

Is the chip powered?

Every 74HC chip and the NE555 have VCC and GND as real pins, and they do nothing until both are wired. Unpowered, every output is high impedance, which looks exactly like a chip that has failed. A supply of at least 2 V across those two pins is what counts as powered.

The ideal gate and the ideal inverter are the exception: they are teaching parts with no supply pins at all.

Is the LED the right way around?

An LED is a diode. Below its forward voltage it barely conducts, and backwards it does not conduct at all. Rotate it with R and try again.

A seven-segment display has the same trap in a different shape: a common-anode part wired the common-cathode way lights nothing.

Is it on the breadboard the way you think?

A breadboard strip is five holes. Two pins in the same strip are one net; two pins in the same row but on opposite sides of the center channel are not. The rails run the length of each side and are not connected to each other unless you wire them.

A part must be dropped so its pins land in holes. A part sitting near the board is not in it.

Is the layer hidden?

Press L for the layers panel. A hidden wire or a hidden part is still there and still simulating, but you cannot see it. A dimmed layer is easy to mistake for an LED that is off.

Is it actually lit, but dim?

The LED probe is perceived brightness, averaged over about 8 milliseconds, which is roughly what your eye does. A PWM output at a low duty reads dim because it is dim.

Check the numbers

Put the oscilloscope on the node. Two channels on any two nodes, drawn against time while the circuit runs, will tell you in a second whether the pin is moving at all. The signal generator is next to it in the part panel.

Still nothing