My Nissan Micra is a 51 usually a good runner however today after work on my way down the main road which is 60 my car would not gain speed. It would only reach 40 at a push. When I’ve arrived to my destination it smelt. I lifted the bonnet and some smoke came out: not a lot, but enough to question. I’ve checked my oil and it was bone empty. I have topped it up and took it for a run and has picked speed up like normal. I only topped the oil up before Christmas. Is this normal or should I be worried?
tl dr - Obviously, if you are losing all your oil in a month (or so) time frame, it's a bad thing. You need to discover where you are leaking or using the oil and get it fixed.
There are only two ways you are going to be missing oil out of the engine: 1) oil leak; 2) burning oil.
Since you say you only saw a small amount of smoke coming from the engine compartment, I'd suggest you are not burning oil. I think the smoke you are talking about is probably because of excess heat buildup due to the lack of oil in the engine (more on this subject in a minute). When the engine heated up well past normal, any oil on the exterior of the engine (due to leakage) will have started to burn off, causing the smoke. If the engine was burning the oil internally, you'd be seeing voluminous amounts of smoke coming out of the tail pipe, especially considering the amount of time it took for the oil to go away.
The bigger problem here is that you are more than likely losing oil by leakage. Either through a blown seal or a bad gasket somewhere. You may have lost all of your oil at once, which, after you poured in the new amount of oil, should be self-evident where the leak is at, because the new oil will probably be leaking out as well. Look under the car and see where the issue resides. It shouldn't be too hard to self-diagnose and get it to a shop and have them do the work (if you don't feel qualified to do it yourself).
Here is the follow-up to what I was talking about above. The reason you could only go 40mph in your car was that the engine didn't have lubrication, which means it was struggling to maintain the speed you wanted. As the lube is depleted, the engine is running with less and less of the slick stuff it needs to keep going. Had your trip been any longer, the likelihood the engine would have seized is more than a probability, but would quickly have become a reality. There is no doubt there is some level of bearing damage done while running it without the oil in it. How much can only be determined by a complete tear down (which I'm not suggesting at this point). Just know that the engine's ultimate longevity has been shortened by this.