Mac won't restart after updating Tiger

IMAC Power PC G4, 10.4.11, I've owned it since 2002.

For the past several Tiger security updates and using Software Update, after the automatic downloading and installing, the IMAC can't restart - grey screen with rotating dial going endlessly.

I must turn off the power, then turn on and IMAC then starts up as usual.

Can I fix this easily? So the update goes as usual and IMAC then starts itself up as before?

Will it likely get worse?