Jupyter Lab Kernels
The kernel in Jupyter Lab controls how each cell is interpreted, and handles the running of the cell accordingly.
If you select the seminar kernel, then the default behavior is that each cell is interpreted as python code. You can write %%R at the start of a cell, to tell the kernel that the cell contains R code. Alternatively, you can write %%sql or %%bash at the start of a cell, to tell the kernel that the cell contains sql or bash code, respectively.
It is possible to interrupt the kernel and/or restart the kernel. Sometimes the kernel will die on its own, for instance, if the amount of memory allocated to the kernel is exceeded. In such a case, you can just restart the kernel and try to run your work again.