First of all, we in Singapore are much "pampered" crowd. Places like US, Europe & AU the option to have "tethering" is a privilage and you'll be paying exorbitant prices (e.g. USD 45 for 4GB/Month). I know some of my friends are not very happy to see SingTel "limited" data bundle which stands at 30GB per month / SGD 15.
Coming to the point, the personal hot spot works on something called "tethering".
There are two different situations where the PHS end up missing.
1.First time usage (just go to settings and enable it)
2. When you change some of the carrier settings or network settings or APN settings
Solution is simple and simply reset the network settings.
Now let me explain why you need to reset. This is a standard defined in 3GPP and whenever the phone communicates to the network, they both try to agree on this option. So when you change the carrier settings/APN settings, the phone tries to get the information from operator network. However this is not really 'authorized' but we'll still do and when the phone tries to get the details from operator network, it ends up disabling the hot spot.
A reset will change the APN back to "e-ideas" with all default settings and Hot spot is back :)