Weld related cracks may happen when you are “just riding along” – but they probably started some time ago. Even if a poor weld was the cause of the problem, there is normally a telltale thin line that forms over a period of time before the final break occurs.
Because tubes are thickest at the weld areas, there may be enough residual strength in the frame to lash the pieces back together.
See if the weld areas will align relatively well, and if so, lash them with inner tube, then rope (inner tube stops the rope from slipping). Remember that some tubes on the bike frame are under tension when you ride it (down tube), and others are under compression (seat tube). Your lashing has to take that into account – will the weld be trying to pull itself apart, or push itself together?
Surprisingly, you may be able to ride the bike after this fix. It will lose a lot of stability, and depending on the quality of your lashing, there’s the potential for sharp tube ends to work loose and impale you at any moment. However, on smooth surfaces, you should be able to coast along.