Does any 1 know what happens when your legs give out and you can't walk anymore and are dizzy & delirious?

my older brother (who is perfectly healthy) just was getting out of bed and went into the kitchen and fell and kept trying to get backup but his legs...i can't really explain it but they wouldn't work right or work at all out of the blue and was hospitalized just 2 day...does any one know what the problem is?