* Do you have 2 volt sources connected directly together? The simulator may not allow it.
* Do you have 2 ground references, either of which is legitimate by itself, but together they produce paradoxical volt levels at nodes? (Try changing ground icons to a voltage source of definite value.)
* Do you have a mosfet gate being driven at a paradoxical volt level outside of its supply rails? This can easily happen with a P-device particularly, if we forget to set the driver (op amp) to produce a usable output range.
* A simulators may not like it when 2 capacitors are in neighboring wires and those wires contain only a capacitor and nothing else.