Cell not executing

I'm really new to Jupyter Notebook. I just started using it for a class assignment we had to complete for this week. Yesterday, I started having trouble executing one of the cells in my notebook. As far as I can tell it doesn't finish executing and just has the [*] symbol next to the In for that cell. I was able to run it all week, but I'm only now having problems. I've reset my kernel many times and restarted my computer. I'm not sure what else to try. Any advice is appreciated! Here's the code for that cell if it helps:

#adding additional empty column using emptydata array created above
#initializing Severity Label array
SevLbl=np.empty([row,1])
#for loop to categorize RUL values and store labels in SevLbl
for x in range(0,row):
while df['RUL'][x]<50:
SevLbl[x]=[0]
while 125>df['RUL'][x]>=50:
SevLbl[x]=[1]
while 200>df['RUL'][x]>=125:
SevLbl[x]=[2]
while df['RUL'][x]>=200:
SevLbl[x]=[3]
df['SeverityLabel']=SevLbl
#preview new data frame below
df.head(3)

/r/IPython
https://redd.it/7qf80w